Reduction of cuprous cyanide with hydrogen in a liquid medium

ABSTRACT

Cuprous cyanide is dispersed in a high boiling hydrocarbon oil nonreactive with hydrogen, copper or HCN, such as a paraffin mineral oil, and hydrogen is passed through at temperatures from 190* C. to temperatures at or a little below the boiling point of the oil. With an oil boiling above 325* C., suitable temperatures are from a little below 300* C. to approximately 325* C. Hydrogen is passed through at a rate and for a time to produce a substantial excess, for example at least 50 percent excess, and the cuprous cyanide is reduced almost quantitatively to a very pure copper powder. After reduction is complete, the copper powder is separated from the oil, the last traces of oil being removed by known means, such as solvent washing, or removed when the copper is melted. The excess hydrogen carries with it HCN formed in the reduction, and the two are separated and reused.

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[72] Inventor Irving Ruhak 3,515,539 6/1970 Wethern 75/.5 A Bronx, N.Y.3,532,490 10/1970 Burkin 75/.5 A [21] 862805 Primary Examiner-L. DewayneRutledge [22] Filed Oct. 1, 1969 ASSISMIII Examiner-W. W. Stallard [45]Patented 1971 Attorneys-Robert Ames Norton and Saul Leitner [73]Assignee Treadwell Corporation New York, N.Y.

ABSTRACT: Cuprous cyanide is dispersed in a high boiling 54 REDUCTION OFCUPROUS CYANIDE WITH hydrocarbon Oil nonreactive with hydrogen, copperor HCN, HYDROGEN IN A LIQUID MEDIUM such as a paraffin mineral oil, andhydrogen is passed through 10 ClaimsZ Drawing Figs at temperatures from190 C. to temperatures at or a little below the boiling point of theoil. With an oil boiling above [52] 11.5. Cl 75/108, 325 Suitabletemperatures are from a little below 300 C. 75/-5 to approximately 325C. Hydrogen is passed through at a rate [51] llnt. Cl ..C22b 15/08, andfor a time to produce a substantial excess for example at C221) 15/12least 50 percent excess, and the cupro'us cyanide is reduced al- [50]Field of Search IS/.5 A, most quantitatively to a very pure copperPowder. After l m8 reduction is complete, the copper powder is separatedfrom [56] References Cited the 011, the last traces of 011 being removedby known means, such as solvent washing, or removed when the copper isUNITED STATES PATENTS melted. The excess hydrogen carries with it HCNformed in 2,740,708 4/1956 Papee 75/.5 A the reduction, and the two areseparated and reused.

1 FCUPROUS cum/05 l '1 "I HYDROGEN MAKE UP F e HYDROGEN- 55PM REUSE 9 H2H547 6 EXCHANGE? OIL TAKE OFF O/L COPPER POWDER HYDROGEN PAIENTEUum 2 6I97! 3,61 5 3 64 SHEET l 0? 2 Few/mus cYA/v/0 HYDROGEN HC/V MAKE up EFOR MWROGEN 5 PARATOR REUSE 9 H2 7 HEAT 6 EXCHANGER 2 0/L TAKE M OFF0//. 5 COPPER POWDER HYDROGEN fifi. I

INVENTOR.

IRVING L. RUBAK ATTORNEY PATENTEUUET 2s IHTI SHEET 2 UF 2 REDUCTION OFCUPROUS CYANIDE WllTll'll HYDROGEN IN A LIQUID MEDIUM RELATEDAPPLICATION AND BACKGROUND OF THE INVENTION in the patent to RobertsU.S. Pat. No. 3,32 l ,303 issued May 23, 1967, and assigned to theTreadwell Corporation, the assignee of the present application, there isdescribed a process for producing pure copper from solutions of coppersalts, particularly cupric sulfate. The process is carried out in twosteps: Cuprous cyanide is first produced, preferably in strongly acidsolution, by reaction of HCN in the presence of a reducing agent, suchas sulfur dioxide or finely divided metallic copper, which may be aportion of the copper produced in the process. This step of the processdepends on the unique property of cuprous cyanide of being insoluble inacid solution and capable of being formed in high, almost quantitativeyields. The second step takes the cuprous cyanide after separation andreduces it with hydrogen at temperatures between l90 and 600 C., thehydrogen being in very substantial excess, usually at least 50 percentor more. The second step is based on the surprising discovery thatalthough at these high temperatures most cyanides react with hydrogen toform products in which HCN is decomposed, in the case of cuprous cyanidethe reaction is substantially quantitative, better than 98 percent, andthe amount of decomposition of HCN is negligible.

In practical commercial operation certain manufacturing problems havearisen which have nothing to do with the basic new process. In thecuprous cyanide reduction step, if the product is in the form of a finepowder, operating steps have to be at moderate rates in order to preventblowing out of the finely divided cuprous cyanide and/or the equallyfinely divided copper which is produced by the reduction with hydrogen.It is, therefore, common to pelletize the cuprous cyanide. Thiseliminates losses of cuprous cyanide blown through by the stream ofhydrogen and permits considerably faster operating rates.

However, even with pellets there are certain limits on the maximum rateat which the operation can be carried out by reason of the fact that ifexternal heating is used for part or all of the heat, the heat transferis not instantaneous and so sets a limit on the rate at which theprocess can be carried out. It is, of course, possible to supply all ofthe heat by preheating the hydrogen. However, this requires moreelaborate heat transfer equipment and produces a relativelyhigh-temperature exit gas of hydrogen and HCN. This heat is normallylost because in separating the hydrogen from the HCN this has to beeffected either by solution in aqueous solutions or by chilling. Theseparated hydrogen, of course, can be preheated to some extent by heatexchange with the hot exit gases, but even so there is a certaineconomic loss, which, although sufficiently low so that the Robertsprocess represents a real economic advance in savings in the cost ofproducing copper, still the small costs due to heat loss are of economicsignificance in huge commercial plants treating many hundreds of tonsper day.

Another factor of small increased cost is the fact that the cuprouscyanide was produced in an aqueous solution and the water had to beremoved; and if pelletizing is used, pellets may be wet, and there arewater seals in the equipment reducing the cuprous cyanide to copper andHCN, which seals are normally necessary because of the highly toxicnature of the HCN produced. This amount of moisture also represents heatlosses.

SUMMARY OF THE INVENTlON it is with certain improvements in themanufacturing operation of the second step of the Roberts process,namely the reduction of cuprous cyanide with hydrogen at hightemperatures, that the present invention deals. The invention is, ofcourse, not concerned with the past history of the cuprous cyanide, butthe first stop of the Roberts process is a very desirable source of thecuprous cyanide for the present process.

Essentially, the present invention is based on carrying out the hydrogenreduction with cuprous cyanide, preferably in finely divided form,suspended in an oil which is nonreactive with hydrogen, copper or HCN.Paraffin oils are preferred, but the particular source of the paraffinoil is not at all critical. The oil must, however, be of reasonablepurity, that is, it is normally a refined petroleum product sincerefined crude oil contains constituents which are reactive with copper,hydrogen or HCN. The oil must have a boiling point ,sufficiently high sothat under the pressure used it does not boil. Atmospheric pressure isnormally preferred as it avoidsthe complications of operating underpressure.

Heat transfer is by external heating to the high-boiling oil aridthrough it to the cuprous cyanide. This is much more rapid than with thesolid forms of cuprous cyanide, and therefore it is not necessary tointroduce most of the heat by preheating the hydrogen to a high degree,and substantial savings on heating equipment thus become possible. Thisis not to say that the hydrogen should not be preheated, and to theextend that this is feasible by heat exchange with the hot exit gases,it represents a desirable saving, thus further augmenting the operatingeconomies of the process of the present invention.

Another practical operating advantage is that there is no problem withblowing away finely divided cuprous cyanide, and therefore it is notnecessary to form it into pellets, though of course this can be done.The finely divided cuprous cyanide suspended in the oil has a very largesurface to the hydrogen bubbled through and rapid reaction rates areachievable. Finally, there is a further advantage that it is not asnecessary to dry the cuprous cyanide and a damp cake may be used or, ifthe cyanide is introduced through a water seal, any moisture it picks upis rapidly boiled off as the oil is heated up and comes off with theexcess hydrogen and HCN. it is true that the heat to vaporize the wateris still needed, but the heat transfer is so much more efficient andrapid into the oil that this does not involve significant additionalcosts, and the elimination of drying thus represents a further operativesaving. it is preferred to use a product that does not have a highmoisture content. This avoids certain operating problems, such asfoaming, bumping, and the like.

As in any reaction between a gas and solids in suspension, reasonablegood maintenance of a dispersed form is necessary for good outputs. Ingeneral it is preferable to provide some mechanical stirring, but ofcourse the bubbling of the hydrogen through the oil itself effects avery considerable amount of agitation and thus augments any furthermechanical agitation. It is, of course, possible to agitate only by thebubbling of hydrogen through the oil, but in general, since mechanicalagitation is so easily provided, it is preferred to have both forms ofagitation.

While the present invention permits worthwhile operating economies, itdoes not significantly improve the efficiency of the reaction, which isalready substantially quantitative. On the other hand, no adverseeffects are noted, so that the operational savings are obtained withoutany offsetting drawbacks.

It should be noted that not only does the present invention effectimportant manufacturing improvements, but the product produced isactually different from that in the Roberts process. The finely dividedproduct in the Roberts process does not shown a crystal structure byordinary means, Such as X-ray diffraction. This does not necessarilymeans that the powder may not constitute very small crystals. However,they are too small to show up on X-ray diffraction examination. Theproduct of the present invention, however, tends to aggregate and oftenproduces larger aggregates some of which are quite large and have ashiny coating on the outside.

The use of a high-boiling in the present process not only improves heattransfer but also makes for other equipment advantages. As air had to beexcluded, powder feed normally required water seals or similar types ofprotection. However, the suspended cuprous cyanide in oil can bedirectly pumped into reacting equipment and so effects significanteconomy therein while still avoiding the introduction of air.

BRIEFDESCRIPTION OF THE DRAWINGS FIG. I is a semidiagrammatic showing ofa batch operation with certain portions of the apparatus shown insection, and

FIG. 2 is a purely diagrammatic representation of a series of reactorsthrough which the oil suspension of cuprous cyanide passes, thuspermitting a continuous process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to describe moresimply the essentials of the process, including certain reactantquantities, it is simpler and easier to describe the process in the formof a batch process, although for large-scale operation the continuousprocess, which will be briefly described below,'presents many operatingadvantages.

FIG. 1 is semidiagrammatic in nature as the equipment used is ofstandard design. The reactor is shown at l as a section through anagitated kettle with a heating jacket 2, agitator 3, and oil level 4.The agitator, as is customary, is driven, and since the drive can beconventional it is not shown. Preheated hydrogen is introduced below thesurface of the oil by a pipe 5 and after bubbling through, leaves bypipe 6 to a conventional hydrogen-HCN separator. As this is a well knownform of apparatus it is shown in purely diagrammatic form, hydrogenleaving through the pipe 7 and I-ICN through the pipe 8. The separatorrequires lowering the temperature, which is effected in a heat exchanger9 through which separated hydrogen passes, being preheated by the hotexit gases in the pipe 6. Cuprous cyanide is first slurried in the oil,such as a paraffin oil, in a mixing tank 10 from which it is pumped intothe reaction vessel by the pump 11. If the operation is carried out as abatch operation, after the reaction is complete most of the oil isdecanted from the copper powder produced, which sinks to the bottom ofthe kettle after agitation ceases. A small amount of oil with the powderis then drained off through the drain 11 and the oil is either removedfrom the copper by washing with a solvent, or if the copper is to bemelted and cast, the small amount of adhering oil can be burned off inthe melting furnace. Whether it is preferable economically to recoverthis small amount of oil or better to burn it off is purely an economicquestion and shows one of the operating advantages of the process, whichis quite flexible.

A typical batch operating example is as follows: 100 parts of cuprouscyanide is introduced into the oil, the agitator started, air removed byblowing nitrogen through, and the oil heated up to about 300C. Hydrogenis then introduced under a slight pressure, for,example about one inchof mercury, and the temperature permitted to climb to 325 C. by reasonofthe heat introduced through the hydrogen and through the heatingjacket of the reaction vessel. The temperature is maintained until thereaction is substantially complete, which may take about 3 hours orslightly more. Just under 30 parts of HCN is produced, representing aconversion of about 98 percent.

After the reaction is completed, agitation is stopped, most of the oildecanted off, and a small amount of oil with the copper powder whichsank to the bottom of the reactor drawn off and removed by solventcleaning and drying. The pure copper powder produced is a dense andflowable product which shows very high reactivity. The effluent gasescontaining the excess of hydrogen, for example, somewhat over 100percentfare cooled down the two gases separated, and the hydrogenrecycled after preheating, additional makeup hydrogen, ofcourse, beingsupplied.

FIG. 2 illustrates four reactors 12, I3, 14 and 15 in series. As thedrawing is purely diagrammatic since standard forms of equipment can beused, the agitators are not shown, though it is usually desirable toprovide agitation, as is described in FIG. 1. The suspension of cuprouscyanide in oil is introduced through pipe 16 to the bottom of reactionvessel 12. Preheated hydrogen is introduced from a hydrogen manifold 17through a diffuser 18, which is shown purely diagrammatically as it is awell-known design. In each of the reaction vessels 12, 13, I4 and 15 theoil is maintained at a certain level, which is indicated at 19 in allfour vessels. Oil containing partly reacted cuprous cyanide overflowsthrough pipe 20 into the bottom of reactor 13. The hydrogen introducedthrough the diffuser 18 bubbles up and carries off I-ICN to ahydrogen-HCN manifold 21. In reactor 13 the preheated hydrogen isintroduced through a diffuser 22 and the same occurs through diffusers23 and 24 in the other reaction vessels l4 and 15. The reactioncontinues in reactor 13 as has been described in connection with reactor12 and the excess hydrogen carries off I-ICN formed into the manifold21. Further reacted oil CuCN slurry overflows from reactor 13 throughpipe 25 into reactor 14, where the reaction further continues as hasbeen described. Finally, the slurry overflows from reactor 14 throughthe pipe 26 into the bottom of reactor 15, where the same reaction withhydrogen continues as has been described.

The number of reactors in series is not critical but they must besufficient so that the reaction is substantially complete in the lastreactor. Normally there will be about four to six reactors in series. InFIG. 2 the slurry of the reduced copper overflows through pipe 27 wherethe copper is separated from the oil as has been described in connectionwith FIG. 1. The continuous process operates under substantially thesame temperature conditions as has been described in connection withFIG. 1 and the mixture of HCN and hydrogen leaving through the manifold21 is cooled and separated as described in conjunction with FIG. I, thehydrogen being preheated in a heat exchanger which cools down theexhaust gases and permits separation of the I-ICN for reuse. Theseparated hydrogen, of course with additional makeup hydrogen, isrecirculated in the same manner as in FIG. 1 and is preheated in theheat exchanger. This portion of the flow sheet is not changed by thecontinuous process of FIG. 1 and is, therefore, not repeated here.

I claim:

1. In a process for reducing cuprous cyanide at elevated temperatureswith excess hydrogen to metallic copper, separating the hydrogen fromthe hydrocyanic acid produced, and recycling the hydrogen, theimprovement which comprises suspending cuprous cyanide in a high-boilingoil, which oil is inert to hydrogen, copper and HCN, heating up the oiland passing hydrogen in substantial excess therethrough, the temperatureduring the reaction ranging from about 190 C. to a temperature belowthat at which the oil boils under the conditions of reaction, thehydrogen passage being continued until the reduction of the cuprouscyanide to metallic copper,

' is complete, and recovering the copper from the oil and reusing thelatter.

2. A process according to claim I in which the temperature duringreaction ranges from about 300 C. to the temperature below at which theoil boils.

3. A process according to claim 2 in which the oil is a highboilingparaffin oil.

4. A process according to claim 3 in which the reaction of hydrogen withcuprous cyanide is at substantially atmospheric pressure.

5. A process according to claim 2 in which the reaction of hydrogen withcuprous cyanide is at substantially atmospheric pressure.

6. A process according to claim 2 in which the hydrogen and hydrocyanicacid gases produced by the reaction are cooled down and are separatedfrom each other and oil vapor, the cooling being effected by heatexchange with separated hydrogen, whereby the latter is preheated.

7. A process according to claim 3 in which the hydrogen and hydrocyanicacid gases produced by the reaction are cooled down and are separatedfrom each other and oil vapor, the cooling being effected by heatexchange with separated hydrogen, whereby the latter is preheated.

8. A process according to claim 4 in which the hydrogen and hydrocyanicacid gases produced by the reaction are cooled down and are separatedfrom each other and oil vapor, the cooling being efi'ected by heatexchange with separated hydrogen, whereby the latter is preheated.

9. A process according to claim 5 in which the hydrogen and hydrocyanicacid gases produced by the reaction are cooled down and are separatedfrom each other and oil vapor,

the cooling being effected by heat exchange with separated hydrogen,whereby the latter is preheated.

10. A continuous process according to claim 2 in which the suspension ofcuprous cyanide is introduced into a first reaction vessel through whichthe hydrogen is passed. excess hydrogen and HCN are removed, and thesuspension of partially reduced cuprous cyanide in oil is overflowedinto a succeeding reactor through which further hydrogen is passed andoverflow repeated to further reactors in series until the cuprouscyanide is substantially completely reduced in the last reactor and thesuspension of copper in oil is overflowed from the last reactor and thecopper separated and recovered.

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2. A process according to claim 1 in which the temperature duringreaction ranges from about 300* C. to the temperature below at which theoil boils.
 3. A process according to claim 2 in which the oil is ahigh-boiling paraffin oil.
 4. A process according to claim 3 in whichthe reaction of hydrogen with cuprous cyanide is at substantiallyatmospheric pressure.
 5. A process according to claim 2 in which thereaction of hydrogen with cuprous cyanide is at substantiallyatmospheric pressure.
 6. A process according to claim 2 in which thehydrogen and hydrocyanic acid gases produced by the reaction are cooleddown and are separated from each other and oil vapor, the cooling beingeffected by heat exchange with separated hydrogen, whereby the latter ispreheated.
 7. A process according to claim 3 in which the hydrogen andhydrocyanic acid gases produced by the reaction are cooled down and areseparated from each other and oil vapor, the cooling being effected byheat exchange with separated hydrogen, whereby the latter is preheated.8. A process according to claim 4 in which the hydrogen and hydrocyanicacid gases produced by the reaction are cooled down and are separatedfrom each other and oil vapor, the cooling being effected by heatexchange with separated hydrogen, whereby the latter is preheated.
 9. Aprocess according to claim 5 in which the hydrogen and hydrocyanic acidgases produced by the reaction are cooled down and are separated fromeach other and oil vapor, the cooling being effected by heat exchangewith separated hydrogen, whereby the latter is preheated.
 10. Acontinuous process according to claim 2 in which the suspension ofcuprous cyanide is introduced into a first reaction vessel through whichthe hydrogen is passed, excess hydrogen and HCN are removed, and thesuspension of partially reduced cuprous cyanide in oil is overflowedinto a succeeding reactor through which further hydrogen is passed andoverflow repeated to further reactors in series until the cuprouscyanide is substantially completely reduced in the last reactor and thesuspension of copper in oil is overflowed from the last reactor and thecopper separated and recovered.